The present invention refers to an optical sensor for measuring physical quantities, having high rejection of undesired environmental disturbances and noise.
Said sensor converts said physical quantities in changes of the state of polarization of the light propagating within it.
In polarimetric sensors systems, wherein the information concerning the entity of the measured quantity is modulated in the polarization state of the light, the problem exists to control the light polarization both in the sensor and the transmission line of the signal modulated in polarization, which is usually realized by mean of optical fibers.
This effect is increased when optical fibers are used to connect the various optical parts of the system and for realizing the transmission line of the signal modulated in polarization. In fact, environmental parameters, such as vibrations, stress, strain, pressure and temperature variations modify the fiber birefringence and consequently change the state of polarization of the light beam guided in the optical fiber itself. For real in-field operation, for instance in typical industrial environments, of any fiber optic sensor systems, it is therefore necessary to reduce the disturbances and the related measure errors caused the environmental conditions.
Various systems have been proposed in order to avoid or reduce said errors.
For instance, a known approach is that of using special optical fibers, called polarization-maintaining single mode optical fibers. These fibers are able to maintain the state of polarization (linear) of the guided light, but the longer is the fiber length, the lower is the polarization level they allow to maintain.
A second known solution consists in using active or passive compensation methods, being able to take the polarization state of the light in the desired condition, so compensating the undesired environmental noise.
Actively compensated systems are therefore known, which need an optical device, usually powered by electrical signals, for introducing a suitable correction. The amount of the correction is derived from a previous analysis of the signal. However, the necessity of using electric wires for conveying the correction and control signals reduces the electromagnetic insensitivity proper of the method being based on optical sensor only; moreover, the necessity of a first elaboration of the signal for obtaining the necessary correction introduces a reduction of the band passing in the system, due to the analysis times.
Often actively compensated systems are dynamically limited by the maximum error value they can correct.
In document WO95100046 a system is described wherein the light coming out from a Faraday sensor is splitted in two light beams having different linear polarization. Said components are converted in intensity variation of an electric current, and then are normalized and processed for obtaining a final signal that is insensitive to temperature changes of the sensor. However said system realizes an immediate conversion of the optical signal into an electrical signal, which is consequently exposed to electromagnetic noise; also, the system works only for AC measurements.
Passively compensated systems are also known, in which the signal correction is realized by means of particular signal processing and/or optical configurations. For example US.5008611 describes a particular designing method to calculate the proper orientation of the optical means constituting the sensor, so that the effects of the birefringence on the optical means can be minimized with respect to the contribution of the Faraday effect.
To reduce the magnitude of the disturbances, systems are also known that transduce the information using the modulation of the light intensity instead of its state of polarization. In fact, in this case, the light intensity is far less sensitive that the light polarization to the same environmental disturbance. However, the sensor systems that transduce physical perturbations in modulation of the intensity of the light signal are less sensitive than the analogous polarimetric type, and often they are also less linear and present a reduced dynamic range.
Methods are also known for canceling reciprocal disturbances. An optical configuration being particularly interesting for realizing this fact is constituted by a retracing optical circuit. In said retracing optical circuit, the light travels two times, in both directions (forward and return), along the same optical path: an optical fiber with a mirror at one of its ends is a common example of retracing system.
In these optical systems, the perturbation effects acting on the fiber are added during the two travels (forward and return) in a well defined way, which depends either upon the birefringence changes and the initial state of polarization.
It is also known that when a particular device called orthoconjugate reflector replaces the mirror, in every point of the, optical circuit the state of polarization of the counterpropagating light beam is orthogonal with respect to the forward propagating one. In the particular case of an optical fiber, the polarization state exiting the same is orthogonal with respect to the incoming one. Generally speaking, an orthoconjugating reflector is an optical device that reflects back a light beam orthogonally polarized with respect to the incident one. A practical embodiment of such a device is the so-called Mirrored Faraday Rotator (MFR): it is built with a Faraday rotator of 45xc2x0 of rotating power, followed by a mirror. This device compensates every disturbance of whatever magnitude, only if it is reciprocal, i.e. it does not depend on the direction of propagation of the light. A non-reciprocal effect, as the Faraday effect itself, invalidates the ability of this optical circuit to compensate birefringence changes.
The present invention has the purpose of solving the above mentioned drawbacks and to indicate an apparatus being able to measure different physical quantities with high rejection to environmental disturbances.
Within this frame, the aim of the present invention is to realize an apparatus being able to measure physical quantities or perturbations, by transducing them in variations of the state of polarization of an optical signal, and then in variations of the light intensity, in order to reduce the sensitivity of the whole apparatus to environmental disturbances and to the initial state of polarization of the light beam. Another aim of the present invention is to realize an apparatus being able to measure physical quantities or perturbations by transducing them in variations of the state of polarization and thereafter in variations of the light intensity, including a transducer having a high rejection to the environmental noise.
For attaining these aims, the subject of the present invention is an apparatus for measuring physical quantities having the features of the annexed claims.
Further aims, characteristics and advantages of the present invention will result in being clear from the following detailed description and the annexed drawings, which are supplied as an explicative and not limiting example, wherein: